1. Field of the Invention
The present invention relates to the removal of particulate from systems having high flow volume therethrough. More particularly, the present invention relates to Selective Catalytic Reduction (SCR) reactors. Still more particularly, the present invention relates to removal of the accumulation of fly ash on the catalyst used in the reduction process.
2. Description of the Prior Art
Selective Catalytic Reduction (SCR) reactors are being employed in fossil fuel-fired electric utility boilers to reduce nitrogen oxide (NOx) emissions generated in the combustion process for such boilers. These reactors are normally installed downstream of the boiler economizer, just upstream of the air preheaters. Operating temperatures range from 600xc2x0 F.-750xc2x0 F.
Particulate fly ash matter from coal-fired boilers ranges 5%-30% of coal burned in the boiler. The quantity of fly ash particulate from oil and natural gas-fired boilers is substantially less. This particulate matter is generally transferred with the flue gas through one or more systems designed to clean the flue gas prior to emission. Such systems include SCR reactors, air pre-heaters, precipitators, scrubbers, and others well known to those skilled in the art. As a result of the transfer of ash matter and other contaminants, it is not particularly surprising that these systems, to varying degrees, become contaminated with the matter passing through.
For the SCR reactors that are used to remove certain gases from the flue gas, there are typically two to five layers of catalyst beds installed therein to facilitate removal of the NOx emissions. It is to be noted that the flue gas flow through the SCR reactor is normally vertically downward and so the catalyst beds are installed horizontally to allow the passage of the flue gas therethrough. Alternatively, however, the catalyst layers can also be arranged in a vertical fashion inside the reactor to permit horizontal flue gas flow through the SCR reactor.
An ammonia (NH3) injection system is located in the flue gas ducting upstream of many of the currently designed SCR reactors. The introduction of ammonia may have some effect on the overall treatment process when used in conjunction with the present invention. Of course, in those systems where ammonia input is not used, this is of no concern. It is to be noted, however, that chemical reagents alternative to ammonia may be employed to accelerate the catalytic reduction of the NOx. It is intended that the present invention is directed to the broader design of the SCR reactor and is not limited to any one embodiment associated with chemical reagents that may or may not be used.
Continuing with the specific operation of an SCR reactor, during NOx removal, ammonia gas or its alternative, is injected with the flue gas duct and into the SCR reactor vessel. In the presence of that particular reagent, the following catalytic reactions take place, resulting in conversion of NOx compounds in flue gas to harmless nitrogen compounds and water vapor.
4 NO+4 NH3+O2xe2x86x924N2+6 H2O
NO+NO2+2 NH3xe2x86x922 N2+3 H2O
Two types of catalyst beds of defined geometry are generally used in the SCR reactor. The two types typically used are: 1) honeycomb-type (or grid-type) and 2) plate-type. Either of the two catalyst beds is normally assembled into standard commercial-size modules to facilitate loading and handling in approximately half-meter or one-meter increments per layer. The catalyst is suspended within the SCR reactor, ordinarily in a plurality of layers, with the catalyst installed one-half to one-meter in depth per layer.
In an exemplar processing operation, flue gas resulting from a combustion process enters the first catalyst layer at a velocity of about 8-20 feet per second. The flue gas passes through holes (honeycomb-type) or slots (plate-type) in the first catalyst layer, exits the first catalyst layer, enters the second catalyst layer, and so on. Holes or slots (also known as hydraulic diameter or pitch opening) in the catalyst layer are normally about 3 mm to 8 mm, closely spaced. In this manner, 70% to 95% of the catalyst layer surface is open to passage of flue gas through it.
Fly ash particle size distribution and particle sizes are highly dependent on the nature of fuel burned and boiler process conditions. In general however, fly ash particles entering the SCR reactor can range in size from about 0.01 mm to about 3 mm in diameter. However, these particles do agglomerate with each other causing particle sizes of 1 cm or larger to form. Of course, particles larger in size than the available catalyst pitch opening, cannot traverse through the catalyst layer, hence these particles collect and continue to build up upstream of the catalyst layer. Moreover, particles nearly equal in size to catalyst hydraulic diameter often lodge inside the catalyst in the holes or slots.
The agglomeration of particles can have a significant adverse impact on the efficiency of the SCR reactor and the boiler. Specifically, the effective reaction zone of the SCR reactor is diminished and so the reaction time is diminished. This naturally affects the entire energy generation system in an adverse way and so it is undesirable to have a build up of particles in the SCR reactor. Since power generation systems, particularly those including SCR reactors, are designed in fine balance, it is important that all subsystems operate substantially as designed. When the operating conditions of the SCR reactor change, the balance of the entire reaction process and therefore the power generation can be altered adversely. In particular, the flue gas must have enough treatment time to ensure NOx removal in line with the design of that reactor. If the SCR reactor is plugged with agglomeration, that treatment time is not provided and the system passes NOx gasses through the remainder of the system. Moreover, if the SCR becomes partially plugged, the fan system used to move gases through the system may not be adequate to overcome the additional pressure drop through the reactor. In sum, there is a fine balance in the system and plugging of the SCR reactor throws that balance off.
Two methods are currently employed to facilitate passage of the flue gas, including the entrained particles, through the catalyst layers. In the first method, traveling sootblowers inject a superheated steam stream of sufficient pressure into the catalyst layers, concurrent with flue gas direction. In the second method, sonic horns are also operated inside the SCR reactor vessels in an effort to excite or vibrate the fly ash particles through the catalyst openings using low-frequency sound energy.
Both of the noted existing methods have an important deficiency. Specifically, neither process acts to remove the particles; instead, they are designed to simply agitate agglomerations in an attempt to dislodge them, assuming that the loosened particles will pass through the catalyst beds. This works to various degrees of effectiveness, however, when a hole or slot is plugged, neither of the noted options is substantially successful in unplugging plugged holes and slots. That results in diminished capacity of the SCR reactor in that the number of reaction sites available in the reactor as designed are not contacted by the flue gas and ammonia mixture.
In regard to the traveling sootblowers in particular, steam is typically drawn from system boiler operation to perform the particle dislodging. That operation consumes a significant amount of system steam that would otherwise be employed in the energy generation operation. Therefore, traveling sootblowers diminish the heat rate of the boilerxe2x80x94an undesirable outcome particularly as even very small changes in system efficiencies translate into significant energy product cost increases. Further, the steam which is used, and which must be used with the sootblower, subsequently can condense within the SCR reactor. That water diminishes catalyst effectiveness and life.
As an alternative cleaning arrangement, the SCR reactor can be shut down, allowed to cool, and the particulate manually removed from the catalyst bed. This process may be time consuming and costly in terms of the power generation process.
Therefore, what is needed is a means for removing particulate matter from systems used to transfer large volumes of particulate-containing fluid. What is also needed is a system for removing particulate from such transfer systems that is relatively non-intrusive to the operation of the system being cleaned and that can be accomplished while the system remains online. Further, what is needed is such a cleaning system that is relatively simple to operate. Yet further, what is needed is such a system for use in association with SCR systems.
It is an object of the present invention to provide a means for removing particulate matter from systems used to transfer large volumes of particulate-containing fluid. It is also an object of the present invention to provide a cleaning system for removing particulate from such transfer systems that is relatively non-intrusive to the operation of the system being cleaned and that may be used while the system remains online. Further, it is an object of the present invention to provide such a cleaning system that is relatively simple to operate. Yet further, it is an object of the present invention to provide a cleaning system that may be used in association with SCR systems.
These and other objects are achieved in the present invention, which offers an easier and more reliable method of particulate removal. In particular, the present invention is well suited for fly ash particle collection and removal upstream of each catalyst layer in an SCR reactor.
In one proposed embodiment, the invention includes means for injecting relatively high-pressure gas upward through the catalyst layers counter to the direction of the flow of the flue gas to be treated in the SCR reactor. Although the high-pressure gas may come from some external source, it preferably is obtained from the flue gas downstream of the catalyst layers in order to minimize disruption of the catalytic process within the SCR reactor. That is, the high pressure heated reduced flue gas or, alternatively air when the volume of downstream gas is inadequate, is injected counter to the direction of the flow of the flue gas. On the other side of each catalyst layer, fly ash particulate matter is vacuumed at the opposite side of each catalyst layer and re-injected into ducting upstream of the primary particulate collection device, such as an electrostatic precipitator or a fabric filter, or it may be injected downstream beyond the catalyst beds for subsequent cleaning. Optionally, a cleaning unit is coupled to the vacuum system so as to enable removal of the vacuumed particles while further optionally enabling return of the removed gas to the SCR reactor.
These and other advantages of the present invention will become apparent upon review of the detailed description, the accompanying figure, and the appended claims.